1. Field of the Invention
The present invention relates to decoding devices and methods, receiving devices and methods, and programs, and particularly to decoding device and method, receiving device and method, and a program that each allow synchronization at high speed even when the frequency spectrum is inverted.
2. Description of Related Art
It is known that, if quadrature amplitude modulation (QAM) as one of multilevel quadrature amplitude modulation schemes is employed, demodulated data involves phase ambiguity of zero degrees, 90 degrees, 180 degrees, and 270 degrees because it is impossible to determine the absolute phase in demodulation processing.
As a related-art scheme for removing such phase ambiguity, there has been proposed e.g. a coded modulation system described in “Rotationally Invariant Convolutional Channel Coding with Expanded Signal Space—Part II: Nonlinear Codes” (IEEE Journal on selected areas in communications, Vol. SAC-2, No. 5, September 1984) (this document will be referred to as Document 1, hereinafter).
In the scheme proposed in Document 1, relative phase data subjected to differential coding is modulated on the transmission side, and demodulated relative phase data is subjected to differential decoding so as to be converted into absolute phase data on the reception side. This allows achievement of correct data even when the phase is rotationally shifted (90 degrees, 180 degrees, 270 degrees) between the modulation and the demodulation.
Furthermore, as disclosed in Japanese Patent Laid-open No. Hei 9-247226 (referred to as Patent document 1, hereinafter), there has also been proposed a scheme in which, if the relationship between the upper side band and the lower side band of the frequency spectrum in demodulation is opposite to that in modulation, the in-phase axis and the quadrature axis are switched to thereby absorb ambiguity of the quadrature phase and thus stably carry out demodulation. Moreover, there has also been proposed a scheme in which, for the case of a signal constellation of rotational symmetry, the bit configuration of data is changed to thereby achieve an effect equivalent to that of the state in which the in-phase axis and the quadrature axis are interchanged.
That is, the technique of Patent document 1 has been proposed in order to solve the problem that it is impossible for the technique of Document 1 to achieve correct data if the frequency spectrum is inverted between modulation and demodulation.